CESM Ocean Component (POP2) Metrics & Diagnostics

Current Practice

The CESM Ocean Model Working Group (OMWG) has diagnostics packages to assess both spatial and temporal characteristics of ocean model simulations. We perform such assessments routinely for output from both forced ocean and fully-coupled experiments. The use of these packages includes evaluating the validity and climate state of a particular coupled model simulation, assessment of whether new capabilities in any component model, when coupled to the entire coupled model system (i.e., a new model code base), have impacted the ocean, and validating long coupled control simulations. The OMWG also employs many different metrics and diagnostics when implementing new capabilities and parameterizations into the ocean component for their particular assessments. These diagnostics are usually very specific for the particular new model additions and are tested within forced ocean-only or forced ocean – sea-ice simulations. The following table is a summary of the most commonly used metrics and diagnostics that are considered. However, we note that many other metrics and diagnostics for additional fields as well as expert judgement are also used in our assessment of a given model simulation.

Current Metrics and Diagnostics for Coupled Model Assessment

Metric/Diagnostic Name

Assessment Method

Assessment Criteria

Assessment Type

Sea surface temperature (SST)

Spatial distributions; mean and RMS of global differences from obs

mean near zero; RMS near zero; both comparable to or better than previous versions; small west coast biases; spatial structure; etc.

Climatological mean over 20+ years

Sea surface salinity (SSS)

Spatial distributions; mean and RMS of global differences from obs

mean near zero; RMS near zero; both comparable to or better than previous versions; small local biases; spatial structure; etc.

Climatological mean over 20+ years￼￼￼￼￼

Global and Atlantic meridional overturning circulation (MOC)

Spatial distributions; time series

No collapse; “reasonable” structure; if possible, comparison with the RAPID array

Consistent with surface fluxes; similar to obs in 20th century simulations

Time series

Heat and salt budgets

Ocean heat and salt contents; surface fluxes

Consistency of fluxes across components; conservation of heat and salt

￼￼Time series and time averages over given time segments

￼￼￼￼￼Meridional heat transport

￼￼￼￼￼￼Transport distribution; maximum values

￼￼￼Compare to present-day obs; compare to RAPID array

￼￼￼￼￼￼Climatological mean over 20+ years

￼￼￼Desired Redesign & Improvements

Our current diagnostics packages and assessment metrics date back to more than 10 years ago. These tools grew out of a few individual people’s own research analysis and plotting codes. As such, they are not really designed to handle continuously growing needs of our expanding modeling community. Incorporation of new diagnostics tools, new analysis methods, new metrics, any changes in the CESM data output structure, etc. are not as straight-forward as we would like. Therefore, we would like to redesign our metrics and diagnostics packages, starting at the upper-most level and considering our anticipated needs. We note that some of the desired diagnostics also require code additions and modifications in the CESM ocean component. However, the path forward will be influenced by our limited resources as well as possible development of a similar package for the MPAS-Ocean model that is being considered as a future CESM component. The following bullet items list only some of the many additional, new assessment diagnostics that need to be developed to meet our future needs.

Desired Metrics and Analysis

Water mass analysis to assess i) atmosphere-ocean partitioning of carbon and heat on decadal to centennial timescales, ii) global and basin scale distributions of volumetric temperature and salinity census. Among others, we will use water mass transformation function.